First thing I do is take a very bright light (LED Flashlight) and shine it through the stones from several angles. Fractures show up well this way. I have cut hundreds of Idaho Star Garnets from numerous locations including the BLM site. The rutile that makes the stars may be dense in the center, the outside or edge to edge from different locations. So there is not one test that tells all for how good the stone is. I have even cut some stones that had no translucence and gotten brilliant stars grinding from the outside. The stones from the BLM site are generally best cut from the center out.
Your top stone appears very solid but is cubic and most Idaho stones are Dodechehedron ( has a lot more faces). I would certainly try a quick polish on a corner to test color,etc. The second stone has a lot of grainy areas and layering. This may be what is considered a sand garnet or just not as high a quality. That does not mean it is not worth trying. This is the type of stone that many cutters would tap with a hammer before trying to cut. I do see some red in the photos and that is a good sign.
Use a lot of water and start with a 220 grit or finer wheel. Garnets hate heat and can even explode. good luck, neal
